Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.
Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.
Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-Which reason best explains why reflectin proteins do not adopt a defined tertiary structure?

Question

Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.

Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.

Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-Which reason best explains why reflectin proteins do not adopt a defined tertiary structure?

Quizplus · Accepted Answer

11ecba2d_0eda_87ab_a3bf_e935c3d103d7_MD0008_00 Tertiary structure is the three-dimensional folded form of a protein. It is stabilized by multiple forces, including hydrogen bonding and ionic interactions between side chains, but the primary driving force in protein folding is the hydrophobic effect. Hydrophobic residues cannot hydrogen bond with the water molecules in an aqueous environment, so instead the water molecules surrounding these residues hydrogen bond with each other in a rigid lattice called a solvation layer. These interactions are entropically unfavorable (negative ΔS), and to avoid them, hydrophobic residues become spontaneously buried in the interior of proteins, away from the aqueous environment. The passage states that reflectins have few leucine, isoleucine, valine, phenylalanine, or tryptophan residues, all of which are hydrophobic. Instead, reflectins are rich in hydrophilic arginine and serine residues, which facilitate hydrogen bonding with water molecules. The relative lack of hydrophobic residues means that reflectins only experience a weak hydrophobic effect and therefore do not fold. (Choices A and C) Reflectins are rich in hydrophilic residues such as arginine and serine, which interact strongly with water through hydrogen bonding. (Choice D) It is true that reflectins are largely composed of basic arginine residues, but basic proteins fold readily if they have sufficient hydrophobic regions. It is not basicity that prevents reflectins from adopting tertiary structure. Educational objective: Proteins frequently fold into specific three-dimensional forms called tertiary structures. The major driving force behind folding is the hydrophobic effect, which causes hydrophobic residues to be buried on the interior of proteins, preventing water from forming solvation layers.

Passage Many Cephalopods Can Change Color to Blend in with Their